1. Field of the Invention
The present invention relates to a motor driving apparatus which includes a position detection unit.
2. Description of the Related Art
A common control for a stepping motor is an “open-loop control” which does not have a feedback loop and is operated in synchronization with a command pulse. Such a control allows a digital positioning operation to be performed easily. Due to such characteristics, the open-loop control is widely used in home information appliances, such as cameras and optical disk devices, and office equipment, such as printers and projectors.
However, there is a problem that during high-speed rotation or when a load on the motor is large, a step-out phenomenon occurs in which the motor can no longer rotate following the command pulse.
To resolve this problem, a brushless DC motor in which a magnetic sensor is mounted on a stepping motor to switch energization according to the position of the rotor is known as a technique to prevent the step-out.
However, to efficiently drive the brushless DC motor, the magnetic sensor has to be mounted with precision. The reason for this is as follows.
FIG. 9 is a diagram illustrating a relationship between torque generated when a constant current flows through a coil and a rotor rotation angle.
When the coils of a motor are configured of two (A phase and B phase) coils, the current can flow in the two coils in the positive direction and the reverse direction, respectively. The torque, when positive energization is carried out in the A phase and in the B phase, is expressed as A+B+, and the torque, when reverse energization is carried out in the A phase and in the B phase, is expressed as A−B−.
Under such a condition, torque waveforms like that illustrated in FIG. 9 can be produced. FIG. 9 illustrates the relationship between the rotor angle and the torque generated in the motor based on the four energization patterns of A−B−, A+B−, A+B+, and A−B+.
All of these patterns are waveforms that have the same and a roughly sinusoidal shape, and have a 90° phase difference in terms of their electrical angle.
Here, the term “electrical angle” expresses one cycle of this sine wave as 360°. If a number of poles of the rotor is n, 1° of electrical angle corresponds to (2×actual angle/n).
To rotate the motor, the energization to the coil is successively switched to produce a torque waveform as illustrated by T1 of FIG. 9, whereby a high torque can be constantly obtained.
Timing for switching the energization to the coil is determined by a signal obtained from the magnetic sensor. Therefore, by mounting the magnetic sensor at an optimum position, the energization can be switched at the timing having the best efficiency.
However, if there is an error in the mounting position of the magnetic sensor, a torque waveform as illustrated by T2 of FIG. 9 is produced, whereby problems arise such as a decrease in motor efficiency and occurrence of noise.
Japanese Patent Application Laid-Open No. 5-176486 discusses a configuration which provides a rotor with a main magnetic field region capable of reducing cogging torque and a sensor magnetic field region which facilitates positioning of a magnetic sensor at a predetermined position. This configuration enables assembly of the apparatus with less mounting error in the magnetic sensor position while reducing the cogging torque.
Here, a case where a mounting error of the magnetic sensor is allowed to be at the electrical angle of ±Δθ° will now be considered. FIGS. 8A and 8B are diagrams illustrating the mounting error of the magnetic sensor. When a distance from a rotating shaft to the magnetic sensor is denoted by R (mm), and the number of poles of the magnet is denoted by n, as illustrated in FIGS. 8A and 8B, a mounting error Δx of ±R sin(Δθ×2/n) (mm) can be permitted in a case of a ±Δθ° angle error. Therefore, the smaller the diameter of the motor becomes, or the higher the number of poles is, the greater the precision that is required for mounting the magnetic sensor.
Recently, various devices which are mounted with a motor have been getting smaller and made with higher precision. Thus, a demand for the motor having a smaller diameter and a larger number of poles is increasing. As a result, higher precision is also required in the mounting of the magnetic sensor.
However, for the configuration discussed in Japanese Patent Application Laid-Open No. 5-176486, since the mounting precision of the magnetic sensor is the same as conventional sensors, there is a problem that when the magnetic sensor position is adjusted with high precision, assembly costs of the motor are increased.